The invention relates to a picture display device comprising a picture display tube and a circuit for feeding the heater thereof having a first and a second terminal for connection to an a.c. voltage source, and circuit means whereby the voltage across the heater assumes, at switch-on of the picture display device, a value which is higher than the nominal value and assumes substantially the nominal value after a predetermined warm-up period, these means comprising a positive temperature coefficient thermistor.
Such a circuit with which warming up of the cathode(s) of a picture display tube is accelerated is disclosed in U.S. Pat. No. 3,886,401. In this prior art circuit the heater is connected after a few seconds to the nominal voltage, i.e. the voltage specified by the tube manufacturer as being the average heater voltage in normal operation, and is heated. For this the voltage of the a.c. voltage source is applied to the heater via a resistor. In parallel with the resistor there is a positive temperature coefficient thermistor which is kept warm and, consequently, highly resistive by means of a second positive temperature coefficient thermistor connected between the terminals.
When the picture display device is switched on, the two thermistors are of low resistance so that the voltage across the heater is higher than in the steady state. In practice, the circuit does not operate satisfactorily when the second thermistor is omitted. In the cold state, the first thermistor must have a very low resistance and, consequently, requires a high current to warm up. As the heater also behaves as a positive temperature coefficient thermistor, the resistance variation of which is, in addition, much faster than that of the first thermistor, the final temperature of this heater must be rather high. This is achieved by means of the second thermistor. This results in that much power is dissipated, particularly by the second thermistor which is connected directly to the voltage from the source. In addition, such a double thermistor is an expensive component.